A number of films composed mainly of plasticized polyvinyl chloride and formed by calendering or casting method have conventionally been employed as a base film used for medical adhesive tapes such as first-aid adhesive tape and rolled adhesive plaster.
The films composed mainly of plasticized polyvinyl chloride are characterized in that when they are left under tension, a high stress at the initial stage relaxes drastically with the passage of time. When a medical adhesive tape composed of a film having such a characteristic is applied to the skin, tensile stress is relaxed gradually after application, resulting in the relief of the burden to the skin.
In recent years, however, it has been requested to take countermeasures against the use of plasticized polyvinyl chloride from the viewpoint of environmental pollution caused by a phthalic acid liquid plasticizer used for it or chlorine contained in it. In addition, it has been pointed out that a large amount of a liquid plasticizer added to plasticized polyvinyl chloride to impart flexibility thereto transfers into a pressure-sensitive adhesive, thereby lowering the cohesion of a pressure-sensitive adhesive layer and causes problems such as so-called adhesive residue and deterioration of pressure-sensitive adhesive force.
A thermoplastic resin having both flexibility and stretch property is under brisk development as a substitute for polyvinyl chloride not only in the medical field but also in another field.
Examples of such a substitute include ethylene-methacrylate-based resin, poly-α-olefin-based resin, ethylene-vinyl acetate-based resin, polyurethane-based resin, and low-density polyethylene-based or linear low-density polyethylene-based resin. Moreover, a medical adhesive tape having a pressure-sensitive adhesive laminated over a substrate base material, which has been imparted with flexibility and strength by incorporating, as a modifier, a different kind of a thermoplastic elastomer in such a resin, is now studied and developed.
Under the present situation, however, a medical adhesive tape produced using a single film or blend film composed of such a resin has not yet fully reached the level of a medical adhesive tape composed of a plasticized polyvinyl chloride (plasticized polyvinyl chloride) film well-balanced in heat resistance, tensile strength, flexibility and stress relaxation.